# std::ranges::partial_sort

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< cpp‎ | algorithm‎ | ranges

Defined in header `<algorithm>` |  |
Call signature |  |
template< std::random_access_iterator I, std::sentinel_for<I> S, class Comp = ranges::less, class Proj = std::identity > requires std::sortable<I, Comp, Proj> constexpr I partial_sort( I first, I middle, S last, Comp comp = {}, Proj proj = {} ); | (1) | (since C++20)
template< ranges::random_access_range R, class Comp = ranges::less, class Proj = std::identity > requires std::sortable<ranges::iterator_t<R>, Comp, Proj> constexpr ranges::borrowed_iterator_t<R> partial_sort( R&& r, ranges::iterator_t<R> middle, Comp comp = {}, Proj proj = {} ); | (2) | (since C++20)

1) Rearranges elements such that the range `[`first`,` middle`)` contains the sorted middle - first smallest elements in the range `[`first`,` last`)`.
The order of equal elements is not guaranteed to be preserved. The order of the remaining elements in the range `[`middle`,` last`)` is unspecified.
The elements are compared using the given binary comparison function comp and projected using proj function object.
2) Same as (1), but uses r as the range, as if using ranges::begin(r) as first and ranges::end(r) as last.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

### Parameters

first, last | - | iterator-sentinel defining the range to sort
r | - | the range to sort
middle | - | the iterator defining the last element to be sorted
comp | - | comparator to apply to the projected elements
proj | - | projection to apply to the elements

### Return value

An iterator equal to last.

### Complexity

\(\scriptsize \mathcal{O}(N\cdot\log{(M)})\)𝓞(N·log(M)) comparisons and twice as many projections, where \(\scriptsize N\)N is ranges::distance(first, last), \(\scriptsize M\)M is ranges::distance(first, middle).

### Possible implementation

    struct partial_sort_fn
    {
        template<std::random_access_iterator I, std::sentinel_for<I> S,
                 class Comp = ranges::less, class Proj = std::identity>
        requires std::sortable<I, Comp, Proj>
        constexpr I
            operator()(I first, I middle, S last, Comp comp = {}, Proj proj = {}) const
        {
            if (first == middle)
                return ranges::next(first, last);
            ranges::make_heap(first, middle, comp, proj);
            auto it {middle};
            for (; it != last; ++it)
            {
                if (std::invoke(comp, std::invoke(proj, *it), std::invoke(proj, *first)))
                {
                    ranges::pop_heap(first, middle, comp, proj);
                    ranges::iter_swap(middle - 1, it);
                    ranges::push_heap(first, middle, comp, proj);
                }
            }
            ranges::sort_heap(first, middle, comp, proj);
            return it;
        }

        template<ranges::random_access_range R, class Comp = ranges::less,
                 class Proj = std::identity>
        requires std::sortable<ranges::iterator_t<R>, Comp, Proj>
        constexpr ranges::borrowed_iterator_t<R>
            operator()(R&& r, ranges::iterator_t<R> middle, Comp comp = {}, Proj proj = {}) const
        {
            return (*this)(ranges::begin(r), std::move(middle), ranges::end(r),
                           std::move(comp), std::move(proj));
        }
    };

    inline constexpr partial_sort_fn partial_sort {};

### Example

Run this code

    #include <algorithm>
    #include <functional>
    #include <iostream>
    #include <string>
    #include <vector>

    void print(const auto& v)
    {
        for (const char e : v)
            std::cout << e << ' ';
        std::cout << '\n';
    }

    void underscore(int n)
    {
        while (n-- > 0)
            std::cout << "^ ";
        std::cout << '\n';
    }

    int main()
    {
        static_assert('A' < 'a');
        std::vector<char> v {'x', 'P', 'y', 'C', 'z', 'w', 'P', 'o'};
        print(v);
        const int m {3};
        std::ranges::partial_sort(v, v.begin() + m);
        print(v), underscore(m);

        static_assert('1' < 'a');
        std::string s {"3a1b41c5"};
        print(s);
        std::ranges::partial_sort(s.begin(), s.begin() + m, s.end(), std::greater {});
        print(s), underscore(m);
    }

Output:

    x P y C z w P o
    C P P y z x w o
    ^ ^ ^
    3 a 1 b 4 1 c 5
    c b a 1 3 1 4 5
    ^ ^ ^

### See also

ranges::partial_sort_copy (C++20) | copies and partially sorts a range of elements (algorithm function object)
ranges::sort (C++20) | sorts a range into ascending order (algorithm function object)
ranges::stable_sort (C++20) | sorts a range of elements while preserving order between equal elements (algorithm function object)
ranges::nth_element (C++20) | partially sorts the given range making sure that it is partitioned by the given element (algorithm function object)
ranges::make_heap (C++20) | creates a max heap out of a range of elements (algorithm function object)
ranges::pop_heap (C++20) | removes the largest element from a max heap (algorithm function object)
ranges::push_heap (C++20) | adds an element to a max heap (algorithm function object)
ranges::sort_heap (C++20) | turns a max heap into a range of elements sorted in ascending order (algorithm function object)
partial_sort | sorts the first N elements of a range (function template)

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